tracker: support metadata request with NULL media
tracker: support metadata request with NULL media

Only with per source device

Signed-off-by: Lionel Landwerlin <9463032a5b99ffad10f36631678a833369a311b5@linux.intel.com>

vrutkovs/grilo-plugins,MathieuDuponchelle/grilo-plugins,kyoushuu/grilo-plugins,MathieuDuponchelle/grilo-plugins,kyoushuu/grilo-plugins,MikePetullo/grilo-plugins,vrutkovs/grilo-plugins,vrutkovs/grilo-plugins,GNOME/grilo-plugins,jasuarez/grilo-plugins,GNOME/grilo-plugins,vrutkovs/grilo-plugins,grilofw/grilo-plugins,jasuarez/grilo-plugins,MikePetullo/grilo-plugins,kyoushuu/grilo-plugins,grilofw/grilo-plugins,MathieuDuponchelle/grilo-plugins,MathieuDuponchelle/grilo-plugins,MikePetullo/grilo-plugins